The security of our nation demands the ability to detect uranium or plutonium in cargo containers entering our borders and the ability to protect against the production of uranium or plutonium in foreign nuclear reactors. Nuclear reactor fuel rods contain the isotopes uranium-238 (U-238) and uranium-235 (U-235). Inside a reactor core, these isotopes absorb neutrons and undergo fission, producing antineutrinos with each decay. Some U-238 isotopes capture neutrons and decay into isotopes of plutonium-239, which also fission and emit antineutrinos. However, the decay of Pu-239 produces substantially fewer antineutrinos than does the decay of U-235 within the energy range required for detection. For a given fuel type, the degree of neutron irradiation primarily determines these changing amounts of fissile material, and is referred to as the “burnup.” The fuel burnup at discharge directly relates to the amount of plutonium in spent fuel, and is an important parameter in the context of nuclear reactor safeguards. Over the course of a nuclear reactor's fuel cycle, the antineutrino count rate drops as uranium content decreases and plutonium increases. In addition, the antineutrino count rate is proportional to the fission rate of the isotopes and, thus, is approximately proportional to the nuclear reactor's power. Nuclear reactors are a major source of human-generated antineutrinos.
Improved detection of neutrons and antineutrinos is needed. For neutron detection, the growing shortage and increasing expense of helium-3 (He-3), which is a light, non-radioactive isotope of helium and the most important isotope in instrumentation for neutron detection, is problematic. He-3 is used in conventional neutron detectors because it has a large capture cross-section for neutrons. When a neutron meets a He-3 atom, they react to form tritium (H-3), which is an isotope of hydrogen with one proton, one electron, and two neutrons, and a hydrogen atom (1H, one proton and one electron), giving off energy in the process. The U.S. Department of National and Homeland Securities ensures the safety of our borders and ports against import of special nuclear material (SNM), such as highly enriched uranium and plutonium. Neutron detection has become increasing difficult because of the worldwide shortage of He-3 due to the nuclear arsenal drawdown at the end of the Cold War. In addition, neutron detection is difficult due to high levels of background noise, high detection rates, and the neutral charge and low neutron energy of the neutrons. There is an urgent need for new neutron detection technologies that have equivalent or higher efficiencies than He-3 in order to replace aging neutron detectors and develop new radiation monitors.
For antineutrino detection, there is also a need for new antineutrino detection technologies. Antineutrinos are electrically neutral, nearly massless fundamental particles produced in large numbers in the cores of nuclear reactors and in nuclear explosions. Because antineutrinos are inextricably linked to the process of nuclear fission, many applications of interest are in nuclear nonproliferation. The emitted antineutrino rate from reactors depends on the thermal power and fissile isotopic content of the reactor. The antineutrino rate can be used to measure the reactor operational status (on/off) and power continuously and in real time. If the reactor power and initial fuel loading are known by other means, and the antineutrino event rates are sufficiently high (roughly, hundreds or thousands of events per day or week), the antineutrino rate can be used to estimate the evolving amounts of fissile uranium and plutonium in the reactor core.
Conventional antineutrino detection technology in the form of liquid scintillation materials has the same drawbacks as with neutron detection, only on a larger scale. Antineutrino detectors are large volume liquid tanks with hazardous and flammable compounds, such as toluene and xylene as well as export controlled and toxic components such as lithium-6 (Li-6). Antineutrino liquid scintillators, like neutron detection scintillators, have issues with regard to size, quantum efficiency, stability, and spatial resolution for the detection area.
Conventional scintillator materials include crystalline materials, plastic materials, or liquid materials. Conventional crystalline scintillator materials, such as LaBr3, CeBr3, or Cs2LiYCl6.Ce (CLYC), are fragile and rather hygroscopic. Conventional crystalline scintillator materials also have high manufacturing costs and size restrictions, lengthy production times to grown the crystals, and are formed from export controlled compounds such as Li-6 and B-10. Conventional plastic scintillator materials suffer from low efficiencies, dead areas due to non-homogeneously distributed components, and low light yields. Conventional liquid scintillator materials contain hazardous, flammable, and toxic materials that can breach and leak, causing a danger to the environment and humans. These properties make crystalline, liquid, and plastic scintillator materials undesirable as a direct replacement for 3He neutron detectors and liquid scintillation antineutrino detectors.